FIGS. 1A to 1F show diagrams illustrating the six main states in which a three-phase brushless DC electric motor (hereinafter referred to as BLDC motor) can be found.
The three-phase BLDC motor 100 includes;                a stator 110 itself including first 111, second 112 and third 113 coils secured to a chassis 114;        a rotor 120 of which the magnet directions are shown by the arrows 121.        
A control circuit including a control circuit and a power circuit (not shown) makes it possible to operate the motor.
Thus, as shown successively by FIGS. 1A to 1F, the rotor 120 is animated by a rotary movement around an axis 130, which causes the motor to successively adopt the six states of FIGS. 1A to 1F, while the stator remains immobile. This rotary movement is initiated and maintained by magnetic fields generated successively in the three coils, with the generation of these fields being controlled by control signals transmitted by the control circuit.
The switching from a current state of the motor 100 to a next state is therefore done by sending, to each coil of the motor, two control signals, by means of the power circuit, with the control signals being dependent on the position of the rotor with respect to the coils of the stator (i.e. the state of the motor).
To determine which control signals must be sent, conventionally, in three-phase BLDC motors, either Hall-effect sensors or a circuit for obtaining synchronisation signals (belonging to the control circuit) have been implemented.
Indeed, the signals from the sensors or the circuit for obtaining synchronisation signals make it possible to have access to the following information:                when it is necessary to switch the motor from one state to another state (transition) by sending adapted control signals;        which control signals must be sent to the coils of the motor (which amounts to knowing which state the motor must switch to).        
It is assumed below that the motor 100 of FIG. 1 is a sensorless motor, i.e. it implements a circuit for obtaining synchronisation signals. The synchronisation signals are obtained on the basis of the measurement of counter electromotive forces generated in the motor.
FIG. 2 shows an example of a circuit 200 for obtaining synchronisation signals of the motor 100 mentioned above (for example the module sold by the ATMEL company under reference AT90PWM3).
The circuit 200 for obtaining synchronisation signals includes first 201, second 202 and third 203 inputs supplied by signals respectively coming from the first, second and third coils of the motor.
It also includes a filtering stage 210 including low-pass filters that make it possible to filter the control signals of the coils and any high-frequency parasitic signals.
It then includes first 221, second 222 and third 223 comparators that respectively compare:                the third input signal 203 and the midpoint of the first 201 and second 202 input signals once filtered by the filtering stage 210;        the first input signal 201 and the midpoint of the third 203 and second 202 input signals once filtered by the filtering stage 210;        the second input signal 202 and the midpoint of the first 201 and third 203 input signals once filtered by the filtering stage 210.        
The outputs of the first 221, second 222 and third 223 comparators respectively constitute first 231, second 232 and third 233 outputs of the circuit 200 on which first, second and third synchronisation signals are respectively delivered.
The circuit 200 for obtaining synchronisation signals is commonly used to synthesise synchronisation signals equivalent to those of the three-phase BLDC motors equipped with Hall-effect sensors.
FIG. 3 shows first 31, second 32 and third 33 theoretical synchronisation signals from the circuit 200 as a function of time.
Thus, by reading the value of the three synchronisation signals and knowing the direction of rotation of the motor, it is possible, in a transition (identified by a rising edge or a falling edge at the level of one of the synchronisation signals), to determine which state the motor must switch to and therefore which control signals to send to it.
Indeed, the standard processing of transitions (implemented by the conventional control circuit) includes the following steps:                monitoring the occurrence of transitions, each being identified by a rising edge or a falling edge, on each of the three synchronisation signals;        detecting an edge (representing a transition) on one of the synchronisation signals;        when the transition is detected, reading the current state of each of the three synchronisation signals so as to determine (knowing the direction of rotation of the motor) the position of the rotor with respect to the stator and therefore which control signal values to send to the coils of the motor;        sending the appropriate control signals to the coils of the motor so as to switch the motor from the current state to the next state;        optionally masking, during a small time zone just after the control signals are sent (so as to avoid interference associated with the switching in progress), the synchronisation signals.        
A problem with this mode of processing transitions is related to the fact that the synchronisation signals are parasitic (or disrupted):                on the one hand, by the chopping frequency of PWM (Pulse Width Modulation) controls;        on the other hand, by the current peaks and/or the demagnetisation peaks associated with the effect of the controls in particular on the coils of the motor.        
Indeed, even if the interference of synchronisation signals associated with the chopping frequency can be limited by the filtering stage 210 of the circuit 200, the current and/or demagnetisation peaks of the coils are almost impossible to filter.
As shown in FIG. 4 (which shows first 41, second 42 and third 43 real synchronisation signals from the circuit 200 as a function of time), these current and/or demagnetisation peaks disrupt the synchronisation signals generated. Indeed, this interference of the synchronisation signals is characterised by the appearance of parasitic peaks 44 that lead in particular to erroneous transition detections and/or erroneous control signal determinations.
The parasitic peaks 44 are dependent on electric parameters (in particular the resistance and the inductance) of the motor and can be present in each of the states of the motor.